The bulk of the interest in crystal technology today, particularly with respect to those crystals used in optical and electronic applications, is focused on monocrystalline bodies obtained from polycrystalline ones. The instant invention, concerned with scintillation crystals, is oppositely directed, namely, to the fabrication of polycrystalline bodies from monocrystalline ones.
The scintillation counter has become a familiar instrument for the detection of many types of ionizing radiation in a wide variety of applications. This growth has been accelerated by the inherent advantages of this type of detector over other means of measurement. These advantages include high sensitivity of gamma rays, availability in a relatively wide range of physical sizes, response proportional to the incident radiation, rapid response time, and fast decay times. Such characteristics have made this type of detector useful in geophysical surveys of uranium and oil, clinical measurement of radio isotopes, radiation monitoring of personnel exposure, as well as many applications in nuclear physics and research.
The basis of a scintillation counting system is the ability of the phosphor to convert into light emission some fraction of the energy lost by ionization during the passage of a charged particle through the material. This emitted light is picked up by the sensitive photocathode of a photomultiplier tube, producing an electrical pulse which can be similar to the light output from the crystal in both magnitude and duration. Depending upon the amplification achieved by the phototube alone, this pulse may be of sufficient size to activate a scaler or rate meter directly, or may require external amplification. For example, a large scintillation phosphor is used as a camera plate coupled to a plurality of photomultiplier tubes used for the detection of ionizing radiation in connection with the analysis of gamma radiation emanating from patients who are injected with specific tracer isotopes.
In general, there are four categories of phosphors used for beta-gamma radiation detection: inorganic crystals, organic crystals, plastic phosphors, and liquid phosphors. The instant invention deals specifically with inorganic crystals, particularly because of their desirable luminescent characteristics, which include a higher density for greater absorption of gamma rays, increased pulse height for detection of low energy interactions, and decay times short enough for fast counting. More particularly, since certain doped alkali metal halides and alkaline earth metal halides exhibit such desirable characteristics as high density, high light output, transparency, and suitable index of refraction, it is with these inorganic crystals that the instant invention is concerned.
More recently, the relatively wide range of physical size referred to hereinabove, in which inorganic scintillator crystals are available, has been found not to be wide enough. The exacting requirements of camera plates, windows, domes, and lenses large enough for modern commercial and military requirements are drawn to crystals substantially in excess of one foot in at least one dimension. Large scintillators in this size range are presently formed from a macrocrystal, which may be either a single crystal or a section of a multiple crystal melt-grown ingot. An ingot is grown in a diameter large enough to yield the scintillation phosphor desired, and sections of the ingot in the desired thickness are sliced from the ingot. Where the dimensions of the desired scintillation phosphor are in excess of those of the ingot that can melt-grown, sections of the ingot are sliced and then adhesively bonded together to from a composite. Attempts to form scintillation phosphors by hot-pressing a polycrystalline aggregate such as a finely divided, pure powder of an ionic salt in the manner described in U.S. Pat. No. 3,359,066 have been conspicuously unsuccessful. Thus, as of the present time, there is no substitute for the arduous devotion which is a perquisite of successful fabrication of a large scintillator by one of the methods described hereinabove. Even so, the end result is a scintillator of determined fragility and, particularly when it is a composite, of scintillation properties which clearly redound to the fact that it is a composite, and therefore suffers from the drawbacks of degradation of light output due to the optical interfaces. No matter how carefully the faces of sections are polished before they are bonded into a composite, there is no known way of eliminating the undesirable effects of the interface. Most importantly, a macrocystal scintillation phosphor, whether a single crystal, a section of a meltgrown multiple crystal ingot, or composite, is prone to cleavage from thermal or mechanical shock, particularly if it is jarred or jolted in a favorable direction. The scintillation phosphors of the instant invention, formed by extrusion of a doped alkali metal halide macrocrystal permit the fabrication not only of phosphors of indefinite length and arbitrary cross section but also of phosphors which, because of their polycrystalline structure, are at least twice as strong as the parent macrocrystal.
It is known that extrusion of a sodium chloride, single crystal billet, not a scintillator, fitted tightly into an extrusion chamber and forced through an extrusion die maintained at various temperatures above 300.degree. C., will yield a rod of polycrystalline sodium chloride which is completely clear and free from porosity. ("Mechanical Properties of Polycrystalline Sodium Chloride" by R.J. Stokes, Proceedings of the British Ceramic Society, Vol. 6, page 192, June 1966). This work was done in connection with a study of the mechanical properties of polycrystalline sodium chloride in relation to those of polycrystalline magnesium oxide. MgO has a similar lattice structure but a melting point of about 2650.degree. C., which is so high as to make the direct study of this "more technologically significant material" all but impossible. Reason for the choice of sodium chloride, other than from the strength of ceramics, and the fundamental aspect of understanding the role of grain boundaries and the deformation of solids as a whole, is that it is an ionic solid which is transparent and affords the opportunity for examining grain-boundary interfaces within the solid rather than their intersection with an external surface, as is the case with opaque materials; and, that being a non-metallic solid, it possesses a wide range of crystal structures and shows a wide variety of slip parameters. Ionic solids thus provide a greater choice of materials on which possible correlations between slip mode and polycrystalline deformability can be examined. The remainder of the disclosure is concerned solely with polycrystalline deformability.
In another study entilted "Effect of Temperature on the Deformation of KC1-KBr Alloys" by Stoloff, Lezius, and Johnston (Journal of Applied Physics, Vol. 34, No. 11, pages 3315, 1963), it was shown that pure potassium chloride single crystals and alloys of potassium chloride and potassium bromide containing from 0.6 to 19 molar per cent potassium bromide, and from which single alloy crystals were formed, may be extruded at 500.degree. to 600.degree. C. through a tungsten carbide die with an extrusion ratio of 16:1 to yield polycrystalline rods which have equiaxed structures with an average grain diameter of 0.35 mm. Extrusion at lower temperatures produced duplex, non-equiaxed grain structures, while extrusion at higher temperatures resulted in only a slightly larger grain sizes. Neither potassium chloride nor the alloy crystals are scintillators, and the remainder of the study is devoted to the effect of temperature, crack propagation, and strength of the polycrystalline extrudates obtained.
In still another reference, entitled "The Scintillation Mechanism in Thallium-Activated Sodium Iodide" (Cooke and Palser, IEEE Transactions on Nuclear Science, Vol. NS-11, No. 3, page 15, 1964) a single crystal scintillator was stressed and its transmission and emission characteristics were observed with respect to the plastic deformation which the crystal suffered. It was found that when the plastic deformation exceeded about 15 per cent, the transmission was essentially nullified (see FIG. 5, id.).
Each of the references cited hereinabove recognizes that the single crystal from which the polycrystalline extrudate was formed underwent a general deformation due to the discrete nature of the slip process. Despite this fact, the dopand, present to a minor extent within the crystal structure and without which the ionic salt crystal displays no practical scintillation characteristics, unexpectedly maintains its position within the crystal structure in such a way as to generate light upon exposure to ionizing radiation in the same way, and at least to the same extent as it did in the parent macrocrystal.